Does having objects behind other objects slow render times down in Cycles?
The short answer to your question is Yes, all of the other objects in your scene contribute to the final image and are computed by Cycles.

In a little bit more detail, Cycles is a ray-tracing render engine. This means that it simulates light rays bouncing around your scene. Just like in real life, these rays can bounce multiple times, which means they can go around corners, refract through glass, and influence the scene regardless of whether or not they are in direct view of the camera. For example, look at this simple scene:



Even though the red sphere is out of the camera's view, it still influences the scene, tinting everything red. It even spreads beyond the direct line-of-sight of the sphere, by bouncing off the walls.

So, how does Cycles render everything? It starts by creating a BVH Tree. BVH stands for Bounding Volume Hierarchy. The BVH Tree contains all of the objects in the scene, including particles and hair, and allows Cycles to make a quicker determination of whether or not a ray has intersected with a mesh. 

Cycles classifies its rays in multiple ways. One of these classifications is Direct vs Indirect. Direct rays happen when an object is lit by a lamp or object that is in its direct line of sight. Because of this, Direct rays are only the first bounce of a ray. All other bounces are indirect rays. This indirect lighting is (most of the time) going to be the largest contribution of objects out of the camera view to your scene.

Knowing all this, what is the best way to decrease render times? There isn't one easy answer, but here are a few suggestions. 


Decrease Max Bounces:

This limits the number of times a ray can bounce before it is killed. Decreasing these can minimize the amount of time spent calculating rays in parts of the scene not visible to the camera. This can make your scene darker, though, since it decreases the influence of indirect lighting sources.
Use AO Bounces (in the Scene panel):

Similar to decreasing Max Bounces, this also limits how many bounces a ray can have before it dies. Instead of just killing the ray, though, this does an Ambient Occlusion calculation first. This can have the effect of lightening the scene, since AO passes are usually brighter than normal rendering. 
Use Camera Cull:

This is the most drastic of the options here. It will actually exclude objects outside of the camera view from the BVH Tree. When using this, you have to be careful not to get weird flickers from objects coming in and out of the camera margin. After enabling it in the Scene settings, you have to individually enable culling for each object in the Object settings:

If you want a little more control, you can manually animate the renderability of an object by hovering over the camera icon and pressing I: